Power system

ABSTRACT

A method of operating a power system of a machine is provided. The method may include selectively supplying power from an engine to one or more other components of the machine., including, during at least some load increases on the power system, supplementing power from the engine with power from an additional power source. The method may also include, during at least some load increases on the power system, supplementing power from the engine with power from an additional power source while recirculating exhaust gas through the engine.

TECHNICAL FIELD

The present disclosure relates to power systems for machines and, moreparticularly, to power systems that include an engine.

BACKGROUND

Many machines have a power system with an engine that provides power toperform various tasks. While the engine of such a power system isproducing power, the engine produces exhaust gas, which often includesvarious undesirable emissions, such as NO_(x). The engine of such apower system may produce greater quantities of emissions when loadincreases on the power system put the engine under high load and/or whenthe engine accelerates rapidly than when the engine operates under lightload/steady-state conditions.

U.S. Pat. No. 5,203,311 to Hitomi et al. (“the '311 patent”) shows anengine with an exhaust-gas-recirculation (EGR) system and a method ofreducing NO_(x), emissions from the engine by recirculating exhaust gasthrough the engine with the EGR system. The EGR system shown by the '311patent includes a first duct with a first valve connected between anexhaust system of the engine and an intake passage of the engine. TheEGR system shown by the '311 patent also includes a second duct with asecond valve and a cooler connected between the exhaust system and theintake passage. The '311 patent discloses opening the first valve andclosing the second valve to recirculate exhaust gas exclusively throughthe first duct when the engine is under light load. Conversely, the '311patent discloses opening the second valve and closing the first valve torecirculate exhaust gas exclusively through the second duct when theengine is under full load.

Although the '311 patent discloses recirculating exhaust gas through theengine to reduce NO_(x) emissions, certain disadvantages persist. Forexample, placing high loads on the engine while recirculating exhaustgas through the engine may significantly compromise performance.Recirculating exhaust gas through the engine may impact the amount ofpower the engine can produce. As a result, recirculating exhaust gasthrough the engine with a high load on the engine may impact the abilityof the engine to meet the load. Additionally, recirculating exhaust gasthrough the engine with a high load on the engine may cause the engineto produce undesirably high amounts of hydrocarbon particulateemissions.

The power system and methods of the present disclosure solve one or moreof the problems set forth above.

SUMMARY OF THE INVENTION

One disclosed embodiment relates to a method of operating a power systemof a machine. The method may include selectively supplying power from anengine to one or more other components of the machine. The method mayalso include, during at least some load increases on the power system,supplementing power from the engine with power from an additional powersource while recirculating exhaust gas through the engine.

Another embodiment relates to a power system for a machine. The powersystem may include an engine, an additional power source, andpower-system controls. The power-system controls may be operable toselectively cause the power system to supply power from the engine toone or more other components of the machine. The power-system controlsmay also be operable to, during at least some load increases on thepower system, supplement power from the engine with power from theadditional power source while recirculating exhaust gas through theengine.

A further embodiment relates to a method of operating a power system ofa machine. The method may include selectively propelling the machine atleast in part by supplying power from an engine to a propulsion device,including, when accelerating the machine, adjusting a speed ratiobetween the engine and the propulsion device as necessary to acceleratethe machine at a target rate while maintaining acceleration of theengine within a target range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of one embodiment of a machinethat includes a power system according to the present disclosure; and

FIG. 2 is a flow chart showing one method of operating a power systemaccording to the present disclosure.

DETAILED DESCRIPTION

FIG. 1 illustrates a machine 10 having a power system 12 according tothe present disclosure. Power system 12 may include an engine 14,additional power sources 16, a drive train 18, propulsion devices 20,power-system controls 22, power-consuming devices 21, andpower-consuming devices 23.

Engine 14 may be any type of device operable to produce power bycombusting fuel. For example, engine 14 may be a diesel engine, agasoline engine, a gaseous-fuel-driven engine, or a gas turbine engine.Engine 14 may include combustion chamber(s) (not shown), an intakesystem 24, an exhaust system 26, an exhaust-gas-recirculation (EGR)system 28, and engine controls 30. Intake system 24 may include anycomponents for delivering charge gas to the combustion chamber(s) ofengine 14, and exhaust system 26 may include any components fordirecting exhaust gas from the combustion chamber(s) of engine 14. EGRsystem 28 may include any components operable to direct exhaust gas intointake system 24 to thereby recirculate that exhaust gas through engine14. For example, as FIG. 1 shows, EGR system 28 may include one or morepassages connected between exhaust system 26 and intake system 24 fordirecting exhaust gas from exhaust system 26 to intake system 24. EGRsystem 28 may also include an exhaust-gas-recirculation (EGR) valve 32for controlling whether and/or at what rate exhaust gas recirculatesthrough engine 14.

Engine controls 30 may include any components operable to control one ormore aspects of the operation of engine 14. In some embodiments, enginecontrols 30 may include EGR valve 32, a fuel-metering system 34, and anengine controller 36. Fuel-metering system 34 may include any componentsoperable to control how much fuel engine 14 combusts per unit time.Engine controller 36 may be any type of information-processing device.Engine controller 36 may be operatively connected to EGR valve 32 andfuel-metering system 34 so that engine controller 36 may control howmuch exhaust gas EGR valve 32 allows to recirculate through engine 14and how fuel-metering system 34 delivers fuel to engine 14. Enginecontroller 36 may also be operatively connected to various other controlcomponents, such as other actuators, valves, sensors, and/orcontrollers. For example, an engine-speed sensor 38 may provide enginecontroller 36 a signal indicating the operating speed of engine 14.Additionally, engine controller 36 may communicate with a mastercontroller 40 of power system 12.

Additional power sources 16 may include various types of componentsoperable to supply power to one or more components of power system 12.In some embodiments, additional power sources 16 may include a motor 42,a motor 43, and a power-conversion unit 44. Each motor 42, 43 may be anelectric motor or a fluid-powered motor, such as a hydraulic orpneumatic motor. Power-conversion unit 44 may be any type of deviceoperable to convert mechanical power into another form of power thatmotors 42, 43 can use to produce mechanical power. For example, inembodiments where motors 42, 43 are electric motors, power-conversionunit 44 may be an electric generator. Similarly, in embodiments wheremotors 42, 43 are fluid-powered motors, power-conversion unit 44 may bea fluid pump. Power-conversion unit 44 may be drivingly connected toengine 14 so that power-conversion unit 44 may convert mechanical powerfrom engine 14 into a form that motor 42 can use.

Additional power sources 16 may also include an energy-storage device 46connected to motors 42, 43. Energy-storage device 46 may be any type ofdevice configured to store energy and supply power to motors 42, 43 in aform that motors 42, 43 can use to produce mechanical power. Forexample, in embodiments where motors 42, 43 are electric motors,energy-storage device 46 may be an electrical storage device, such as acapacitor or a battery. Similarly, in embodiments where motors 42, 43are fluid-powered motors, energy-storage device 46 may be a fluidreservoir. Various other power sources may supply the energy stored byenergy-storage device 46. In some embodiments, power-conversion unit 44may be connected to energy-storage device 46 so that power-conversionunit 44 may supply power to energy-storage device 46.

Additional power sources 16 may also include a power-conversion unit 45.Power-conversion unit 45 may be drivingly connected to motor 43.Power-conversion unit 45 may be any type of device operable to receivemechanical power from motor 43 and convert at least a portion of thatpower into another form. For example, power-conversion unit 45 may be anelectric generator or a fluid pump. In some embodiments,power-conversion unit 45 may be configured to convert mechanical powerinto a different form of power than the form of power thatenergy-storage device 46 receives and supplies. For example,energy-storage device 46 may be a fluid reservoir that receives andsupplies pressurized fluid, and power-conversion unit 45 may be anelectric generator. Conversely, energy-storage device 46 may be anelectrical storage device that receives and supplies electricity, andpower-conversion unit 45 may be a fluid pump.

Additional power sources 16 are not limited to those shown in FIG. 1 anddiscussed above. Additional power sources 16 may include other devicesoperable to supply power to motors 42, 43 and power-consuming devices21, 23. For example, in some embodiments, additional power sources 16may include a fuel cell operable to supply electricity to motors 42, 43and power-consuming devices 21, 23. Similarly, additional power sources16 may include an additional engine and power-conversion unit forsupplying power to motors 42, 43 and power-consuming devices 21, 23.Furthermore, additional power sources 16 may include devices other thanmotor 42 for mechanically supplying power to drive train 18. Moreover,additional power sources 16 may omit one or more of motor 42, motor 43,power-conversion unit 44, power-conversion unit 45, and energy-storagedevice 46.

Drive train 18 may include a multiple-ratio transmission 48 and variousother components for transmitting power from engine 14 to propulsiondevices 20. Multiple-ratio transmission 48 may include a rotary inputmember 50 drivingly connected to engine 14, a rotary output member 52drivingly connected to propulsion devices 20, provisions fortransferring power between rotary input member 50 and rotary outputmember 52, and transmission controls 54. Multiple-ratio transmission 48may have any configuration enabling multiple-ratio transmission 48 totransfer power between rotary input member 50 and rotary output member52 at any of a plurality of speed ratios under the control oftransmission controls 54.

In some embodiments, the configuration of multiple-ratio transmission 48may allow adjusting the speed ratio between rotary input member 50 androtary output member 52 through a continuous range. FIG. 1 shows such anembodiment. As FIG. 1 shows, multiple-ratio transmission 48 may includean additional rotary input member 56. Multiple-ratio transmission 48 mayalso include a planetary gear set 58 connected to rotary input members50, 56 and rotary output member 52 in a manner such that the speed ratiobetween rotary input member 50 and rotary output member 52 variescontinuously as a function of the speed of rotary input member 56.Rotary input member 56 may be drivingly connected to motor 42. As aresult, the speed ratio between rotary input member 50 and rotary outputmember 52 may vary continuously as a function of the speed of motor 42.

Transmission controls 54 may include any components operable to controlwhether and/or at what speed ratio multiple-ratio transmission 48transfers power between rotary input member 50 and rotary output member52. In some embodiments, transmission controls 54 may include atransmission controller 60, which may be any type ofinformation-processing device. Transmission controller 60 may beoperatively connected to motor 42 and power-conversion unit 44 so thattransmission controller 60 can control the speed ratio between rotaryinput member 50 and rotary output member 52 by controlling the speed ofmotor 42. Additionally, transmission controls 54 may include variousother control components, such as actuators, sensors, and/or valves,some or all of which may be operatively connected to transmissioncontroller 60.

Multiple-ratio transmission 48 is not limited to the configuration shownin FIG. 1. For example, multiple-ratio transmission 48 may include otherprovisions in addition to, or in place of, planetary gear set 58 fortransmitting power between rotary input member 50 and rotary outputmember 52. In some embodiments, the configuration of multiple-ratiotransmission 48 may provide only a finite set of discrete speed ratiosat which multiple-ratio transmission 48 may transmit power betweenrotary input member 50 and rotary output member 52. Additionally, insome embodiments, multiple-ratio transmission 48 may omit rotary inputmember 56.

Propulsion devices 20 may be any type of device operable to apply powerto the environment surrounding machine 10 in a manner to propel machine10. For example, as FIG. 1 shows, propulsion devices 20 may be wheels.Alternatively, propulsion devices 20 may be track units or other typesof devices configured to apply power to the ground to propel machine 10.In some embodiments, propulsion devices 20 may be propellers or othertypes of devices configured to propel machine 10 using fluid.

Power-consuming devices 21, 23 may include any types of components thatuse power from other components of power system 12 to perform one ormore tasks. Power-consuming devices 21 and/or power-consuming devices 23may include one or more hydraulically and/or electrically poweredimplements, including, but not limited to, excavating implements,hoists, winches, compactors, clamps, hammers, brooms, saws, chippers,grinders, and/or similar devices. In some embodiments, power-consumingdevices 21 may use a different form of power than power-consumingdevices 23. For example, power-consuming devices 21 may use electricity,and power-consuming devices 23 may use hydraulic power. Conversely,power-consuming devices 21 may use hydraulic power, and power-consumingdevices 23 may use electricity.

Power-consuming devices 21 may connect to power-conversion unit 44 andenergy-storage device 46. Power-control components 47 may control theamount of power supplied from power-conversion unit 44 topower-consuming devices 21, and power-control components 49 may controlthe amount of power supplied from energy-storage device 46 topower-consuming devices 23. Power-control components 47 andpower-control components 49 may include any types of components operableto control the flow of power from one device to another, such as valvesand/or electrical power-regulation devices.

Power-consuming devices 23 may connect to power-conversion unit 45 sothat power-consuming devices 23 may receive power from power-conversionunit 45, which power may come from energy-storage device 46 and motor43. Power-control components 51 may control the amount of power suppliedfrom power-conversion unit 45 to power-consuming devices 23.Power-control components 51 may include any types of components operableto control the flow of power from one device to another, such as valvesand/or electrical power-regulation devices.

Power-system controls 22 may include any components operable to controlpower system 12 in the manners discussed hereinbelow. In someembodiments, power-system controls 22 may include engine controls 30,transmission controls 54, an operator interface 62, and mastercontroller 40. Operator interface 62 may include various componentsconfigured to transmit operator inputs to other components of machine10. For example, operator interface 62 may include a throttle 64 andvarious associated components for communicating operator inputs relatingto desired speed and/or acceleration of machine 10 to other components.Additionally, operator interface 62 may include a cruise-control system65 operable to communicate to other components of machine 10 anoperator-determined target travel speed for machine 10. Furthermore,operator interface 62 may include various other operator input devices63 operable to transmit operator inputs relating to desired operation ofvarious power-consuming devices 21, 23.

Master controller 40 may be any type of information-processing device.Master controller 40 may be operatively connected to engine controls 30,transmission controls 54, motor 43, power-conversion unit 44,power-conversion unit 45, and operator interface 62. Additionally,master controller 40 may be operatively connected to power-controlcomponents 47, 49, 51 and/or to some or all of power-consuming devices21, 23 so that master controller 40 may monitor and control power flowfrom power-conversion units 44, 45 and energy-storage device 46 topower-consuming devices 21, 23. Furthermore, master controller 40 may beoperatively connected to various additional sources of information aboutoperating conditions of machine 10. For example, master controller 40may be operatively connected to a travel-speed sensor 66 that providesmaster controller 40 with a signal indicating the travel speed ofmachine 10. Master controller 40 may also be operatively connected tovarious other sources of information, such as other sensors (not shown)and/or other controllers. Accordingly, master controller 40 maycoordinate control of the various systems of machine 10 dependent oninputs from engine controls 30, transmission controls 54, operatorinterface 62, and other sources of information.

Power system 12 is not limited to the configuration shown in FIG. 1. Forexample, drive train 18 may connect engine 14, multiple-ratiotransmission 48, and propulsion devices 20 in different manners thanFIG. 1 shows. Additionally, in some embodiments, power system 12 mayomit drive train 18 and propulsion devices 20. Furthermore, power-systemcontrols 22 may have a different configuration than shown in FIG. 1. Insome embodiments, one controller may replace two or more of enginecontroller 36, master controller 40, and transmission controller 60.

Industrial Applicability

Power system 12 may have application in any machine that requires powerto perform one or more tasks. Power system 12 may provide power topropel machine 10 by supplying power from engine 14 and/or motor 42,through multiple-ratio transmission 48, to propulsion devices 20.Additionally, power system 12 may provide power for performing variousother tasks with power-consuming devices 21, 23 by supplying power fromengine 14 to power-consuming devices 21, 23. Power system 12 may supplypower from engine 14 to power-consuming devices 21, 23 through one ormore path(s) that include some combination of one or more ofpower-conversion unit 44, power-conversion unit 45, motor 43, andenergy-storage device 46.

When power system 12 is supplying power from engine 14 to one or moreother components of machine 10, power-system controls 22 may coordinateoperation of engine 14, additional power sources 16, and drive train 18in various ways to meet the power needs of machine 10. FIG. 2illustrates one control method that power-system controls 22 mayimplement to maintain desirable performance of power system 12 in avariety of circumstances. Principally, the method illustrated in FIG. 2involves continuously recirculating exhaust gas through engine 14 (step68) and using multiple-ratio transmission 48 and additional powersources 16 to buffer increases in speed and load on engine 14.Power-system controls 22 may continuously recirculate exhaust gasthrough engine 14 by holding EGR valve 32 at least partially open sothat exhaust gas may flow from exhaust system 26, through EGR system 28,to intake system 24 and, from there, back through engine 14.

At the same time, power-system controls 22 may make variousdeterminations relating to the power needs of machine 10. Initially,power-system controls 22 may determine a target travel speed and targetacceleration rate for machine 10 (step 70). Power-system controls 22 maydetermine the target travel speed and target acceleration rate based onvarious factors, such as inputs from throttle 64, cruise-control system65, and travel-speed sensor 66. In some circumstances, inputs topower-system controls 22 may indicate no need to propel machine 10,which may cause a target travel speed of zero and a target accelerationrate of zero.

After determining the target travel speed and target acceleration rate,power-system controls 22 may determine the power load on power system 12(step 72). The power load on power-system 12 may include the powerrequired to propel machine 10 at the target travel speed andacceleration rate and the power required by power-consuming devices 21,23.

After determining the target acceleration rate and the power load onpower system 12, power-system controls 22 may determine whether powersystem 12 is meeting the power load (step 74) and the targetacceleration (step 75). Under steady-state conditions, power-systemcontrols 22 will find that power system 12 is meeting the power load andthe target acceleration. Accordingly, in such circumstances,power-system controls 22 may repeatedly reevaluate the target travelspeed, target acceleration, and power load (steps 70, 72).

When the power load increases, power-system controls 22 may find thatpower system 12 is not meeting the power load (step 74). If so,power-system controls 22 may increase the power supplied to thecomponents of machine 10 to meet the increased power load. Power-systemcontrols 22 may increase the power supplied to the components of machine10 by increasing the amount of power engine 14 supplies. Alternatively,using additional power sources 16, power-system controls 22 may increasethe power supplied to the components of machine 10 without increasingthe power engine 14 supplies. In order to determine which of theseapproaches to take, power-system controls 22 may determine whether thetorque output of engine 14 is at the limit of a target engine-torquerange (step 76). The target engine-torque range for engine 14 may be asubset of the full range of torque output that engine 14 can provide.The target engine-torque range may be defined as a fixed numericalrange, or it may be defined as a function of one or more operatingconditions of machine 10, such as engine speed, engine temperature,ambient air temperature, and/or various other factors.

If power system 12 is not meeting the power load (step 74) and thetorque output of engine 14 is not at the limit of the targetengine-torque range for engine 14, power-system controls 22 may firstattempt to meet the power load by increasing the torque output of engine14 (step 78). Power-system controls 22 may do so by adjusting variousaspects of the operation of engine 14, including the rate at whichfuel-metering system 34 delivers fuel. Power-system controls 22 maycontinue increasing the torque output of engine 14 until the power loadis met or the torque output of the engine 14 reaches the limit of thetarget engine-torque range.

If the torque output of engine 14 reaches the limit of the targetengine-torque range (step 76) and the power load is not met,power-system controls 22 may supplement power from engine 14 byincreasing the amount of power supplied to the components of machine 10by additional power sources 16 (step 80). For example, by increasing thepower output of motor 42 with power drawn from energy-storage device 46or another power source that does not derive its power from engine 14,power-system controls 22 may increase the power provided to propulsiondevices 20 without increasing the power that engine 14 supplies.Similarly, by increasing the net power supplied by energy-storage device46 and/or supplying power from another power-source that does not derivepower from engine 14, power-system controls 22 may increase the powersupplied to power-consuming devices 21, 23 without increasing the powersupplied by engine 14. Power-system controls 22 may continue increasingthe amount of supplemental power supplied to the components of machine10 from additional power sources 16 until the power load is met (step74).

If power-system controls 22 determine that power system 12 is notmeeting the target acceleration for machine 10 (step 75), power-systemcontrols 22 may take steps to increase acceleration of machine 10.Power-system controls 22 may increase the acceleration of machine 10 byincreasing acceleration of engine 14 and/or by increasing the speedratio between engine 14 and propulsion devices 20. In order to determinewhether to increase acceleration of engine 14, power-system controls 22may determine whether acceleration of engine 14 is at the limit of atarget engine-acceleration range (step 82). Power-system controls 22 maydetermine the actual engine acceleration using signals from engine-speedsensor 38. The target engine-acceleration range may include rates atwhich engine 14 can accelerate without producing undesirable levels ofemissions. The target engine-acceleration range may be defined as afixed, numerical range, or it may be defined in terms of one or moreoperating conditions of machine 10, such as torque output, engine speed,engine temperature, ambient air temperature, and/or various otherfactors.

If power system 12 is not meeting the target acceleration (step 75) andthe acceleration of engine 14 is not at the limit of the targetengine-acceleration range, power-system controls 22 may increase theacceleration of engine 14 (step 84). Power-system controls 22 mayincrease the acceleration of engine 14 by adjusting various aspects ofthe operation of engine 14, such as the fuel delivery by fuel-meteringsystem 34. Power-system controls 22 may continue increasing theacceleration of engine 14 until the target acceleration is met (step 75)or the engine acceleration reaches the limit of the targetengine-acceleration range.

If engine acceleration reaches the limit of the targetengine-acceleration range (step 82) without meeting the targetacceleration for machine 10 (step 75), power-system controls 22 mayincrease the acceleration of machine 10 by adjusting the speed ratiobetween engine 14 and propulsion devices 20 (step 86). In someembodiments, power-system controls 22 may do so by adjusting the speedratio between rotary input member 50 and rotary output member 52 ofmultiple-ratio transmission 48 through a continuous range. For example,power-system controls 22 may adjust the speed of motor 42 to adjust thespeed ratio between rotary input member 50 and rotary output member 52.Power-system controls 22 may supply any additional power needed tochange the speed of motor 42 from power-conversion unit 44 and/orenergy-storage device 46, depending on whether engine 14 can supplyadditional power to power-conversion unit 44 without exceeding thetarget engine-torque range for engine 14.

Methods according to which power-system controls 22 may operate powersystem 12 are not limited to the examples discussed above in connectionwith FIG. 2. For example, power-system controls 22 may implementdifferent processes for determining whether and to what extent to bufferengine-load and engine-speed increases during load increases on powersystem 12 and acceleration of machine 10. In some embodiments,power-system controls 22 may buffer all engine-load and engine-speedincreases, rather than only those that exceed target engine torque andengine speed ranges. Additionally, power-system controls 22 may omit oneor more of the actions shown in FIG. 2. In some embodiments,power-system controls 22 may buffer engine-load increases, but notengine-speed increases. Conversely, in some embodiments, power-systemcontrols 22 may buffer engine-speed increases, but not engine-loadincreases.

The disclosed embodiments may provide a number of performanceadvantages. The disclosed methods may provide strong response totransient changes in the power needs of machine 10 without imposinglarge transient loads on engine 14 or accelerating engine 14 atundesirably high rates. Recirculating exhaust gas through engine 14while buffering engine-load and engine-speed increases may help suppressboth NO_(x) and hydrocarbon particulate emissions during load increaseson power system 12 and acceleration of machine 10. Reducing NO_(x) andhydrocarbon particulate emissions during load increases and accelerationmay create leeway to improve various other aspects of the performance ofpower-system 12, such as fuel efficiency.

Additionally, buffering engine-load and engine-speed increases withadditional power sources 16 and multiple-ratio transmission 48 mayreduce the size of engine 14 required to meet the power needs of machine10. This may allow downsizing engine 14, which may decrease thecomponent cost associated with engine 14, reduce the weight of machine10, and make it easier to fit engine 14 within the space available onmachine 10. Additionally, downsizing engine 14 may allow downsizingvarious components associated with engine 14. For example, it may allowdownsizing various cooling system components associated with engine 14because downsizing engine 14 may reduce the amount of heat produced byengine 14. Downsizing various components associated with engine 14 mayfurther decrease component costs associated with machine 10, furtherreduce the weight of machine 10, and make it easier to fit the variouscomponents of machine 10 within the space available on machine 10.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the power system and methodswithout departing from the scope of the disclosure. Other embodiments ofthe disclosed power system and methods will be apparent to those skilledin the art from consideration of the specification and practice of thepower system and methods disclosed herein. It is intended that thespecification and examples be considered as exemplary only, with a truescope of the disclosure being indicated by the following claims andtheir equivalents.

1. A method of operating a power system of a machine, comprising:selectively supplying power from an engine to one or more othercomponents of the machine; and during at least some load increases onthe power system, supplementing power from the engine with power from anadditional power source while recirculating exhaust gas through theengine.
 2. The method of claim 1, wherein supplementing power from theengine with power from the additional power source during at least someload increases on the power system includes supplementing power from theengine as necessary to meet power needs of the machine while maintainingtorque output of the engine within a target torque range.
 3. The methodof claim 2, wherein selectively supplying power from the engine to oneor more other components of the machine includes selectively propellingthe machine at least in part by supplying power from the engine to apropulsion device, including, when accelerating the machine, adjusting aspeed ratio between the engine and the propulsion device as necessary toaccelerate the machine at a target rate while maintaining accelerationof the engine within a target range.
 4. The method of claim 1, whereinselectively supplying power from the engine to one or more othercomponents of the machine includes selectively propelling the machine atleast in part by supplying power from the engine to a propulsion device,including, when accelerating the machine, adjusting a speed ratiobetween the engine and the propulsion device as necessary to acceleratethe machine at a target rate while maintaining acceleration of theengine within a target range.
 5. The method of claim 4, whereinadjusting the speed ratio between the engine and the propulsion deviceas necessary to accelerate the machine at a target rate whilemaintaining acceleration of the engine within a target range includesadjusting the speed ratio through a continuous range.
 6. The method ofclaim 4, wherein adjusting the speed ratio between the engine and thepropulsion device as necessary to accelerate the machine at a targetrate while maintaining acceleration of the engine within a target rangeincludes adjusting the speed ratio of a multiple-ratio transmissionthrough a continuous range by adjusting the speed of a motor drivinglyconnected to one or more components of the multiple-ratio transmission.7. The method of claim 1, wherein supplementing power from the enginewith power from the additional power source includes supplying powerfrom an electric motor to a propulsion device.
 8. A power system for amachine, comprising: an engine; an additional power source; power-systemcontrols operable to selectively cause the power system to supply powerfrom the engine to one or more other components of the mobile machine,and during at least some load increases on the power system, supplementpower from the engine with power from the additional power sourcewhilerecirculating exhaust gas through the engine.
 9. The power system ofclaim 8, wherein the additional power source includes an electric motor.10. The power system of claim 8, wherein the additional power sourceincludes a hydraulic motor.
 11. The power system of claim 8, whereinselectively causing the power system to supply power from the engine toone or more other components of the machine includes selectivelypropelling the machine at least in part by supplying power from theengine to a propulsion device, including when accelerating the machine,adjusting a speed ratio between the engine and the propulsion device asnecessary to accelerate the machine at a target rate while maintainingacceleration of the engine within a target range.
 12. The power systemof claim 11, wherein supplementing power from the engine with power fromthe additional power source during at least some load increases on thepower system includes supplementing power from the engine as necessaryto meet power needs of the machine while maintaining torque output ofthe engine within a target range.
 13. The power system of claim 11,further including: a multiple-ratio transmission; and wherein adjustingthe speed ratio between the engine and the propulsion device asnecessary to accelerate the machine at a target rate while maintainingacceleration of the engine within a target range includes adjusting aspeed ratio of the multiple-ratio transmission through a continuousrange.
 14. The power system of claim 11, further including: amultiple-ratio transmission; wherein the additional power sourceincludes a motor drivingly connected to one or more components of themultiple-ratio transmission; and wherein adjusting the speed ratiobetween the engine and the propulsion device as necessary to acceleratethe machine at a target rate while maintaining acceleration of theengine within a target range includes adjusting the speed ratio of themultiple-ratio transmission through a continuous range by adjusting thespeed of the motor.
 15. The power system of claim 8, whereinsupplementing power from the engine with power from the additional powersource during at least some load increases on the power system includessupplementing power from the engine as necessary to meet power needs ofthe machine while maintaining torque output of the engine within atarget torque range.
 16. A method of operating a power system of amachine, comprising: selectively propelling the machine at least in partby supplying power from an engine to a propulsion device, including,when accelerating the machine, adjusting a speed ratio between theengine and the propulsion device as necessary to accelerate the machineat a target rate while maintaining acceleration of the engine within atarget range.
 17. The method of claim 16, wherein adjusting the speedratio between the engine and the propulsion device includes adjustingthe speed ratio through a continuous range.
 18. The method of claim 17,wherein adjusting the speed ratio between the engine and the propulsiondevice includes adjusting a speed ratio of a multiple-ratio transmissionthrough a continuous range by adjusting the speed of a motor drivinglyconnected to one or more components of the multiple-ratio transmission.19. The method of claim 16, further including, when accelerating themachine, recirculating exhaust gas through the engine.
 20. The method ofclaim 16, wherein selectively propelling the machine at least in part bysupplying power from the engine to the propulsion device includessupplementing power from the engine with power from an additional powersource as necessary to meet propulsion needs of the machine whilemaintaining torque output of the engine within a target range.